This invention relates to new crosslinking agents for epoxy resin compositions, more particularly to phosphorus element-containing compounds useful as crosslinking agents for epoxy resin compositions to yield non-halogenated, ignition resistant phosphorus element-containing epoxy resin formulations. Even more particularly, the new phosphorus element-containing crosslinking agents of the present invention are based on isomeric mixtures of tris(2-hydroxyphenyl)phosphine oxides having the following general chemical structure of Formula I: 
wherein R may be independently a hydrogen or a C1-C10 alkyl group such as methyl, ethyl, propyl, butyl, etc. The flame retardant epoxy resin formulations of this invention are advantageously used for making laminates for printed wiring boards and composite materials.
It is known to make electrical laminates and other composites from a fibrous reinforcement and an epoxy-containing matrix resin. Examples of suitable processes usually contain the following steps:
(1) an epoxy-containing formulation is applied to or impregnated into a substrate by rolling, dipping, spraying, other known techniques and/or combinations thereof. The substrate is typically a woven or nonwoven fiber mat containing, for instance, glass fibers or paper.
(2) The impregnated substrate is xe2x80x9cB-stagedxe2x80x9d by heating at a temperature sufficient to draw off solvent in the epoxy formulation and optionally to partially cure the epoxy formulation, so that the impregnated substrate can be handled easily. The xe2x80x9cB-stagingxe2x80x9d step is usually carried out at a temperature of from 90xc2x0 C. to 210xc2x0 C. and for a time of from 1 minute to 15 minutes. The impregnated substrate that results from B-staging is called a xe2x80x9cprepregxe2x80x9d. The temperature is most commonly 100xc2x0 C. for composites and 130xc2x0 C. to 200xc2x0 C. for electrical laminates.
(3) One or more sheets of prepreg are stacked or laid up in alternating layers with one or more sheets of a conductive material, such as copper foil, if an electrical laminate is desired.
(4) The laid-up sheets are pressed at high temperature and pressure for a time sufficient to cure the resin and form a laminate. The temperature of this lamination step is usually between 100xc2x0 C. and 230xc2x0 C., and is most often between 165xc2x0 C. and 190xc2x0 C. The lamination step may also be carried out in two or more stages, such as a first stage between 100xc2x0 C. and 150xc2x0 C. and a second stage at between 165xc2x0 C. and 190xc2x0 C. The pressure is usually between 50 N/cm2 and 500 N/cm2. The lamination step is usually carried out for a time of from 1 to 200 minutes, and most often for 45 to 90 minutes. The lamination step may optionally be carried out at higher temperatures for shorter times (such as in continuous lamination processes) or for longer times at lower temperatures (such as in low energy press processes).
(5) Optionally, the resulting laminate, for example a copper-clad laminate, may be post-treated by heating for a time at high temperature and ambient pressure. The temperature of post-treatment is usually between 120xc2x0 C. and 250xc2x0 C. The post-treatment time usually is between 30 minutes and 12 hours.
It is conventional in the preparation of epoxy-containing laminates to incorporate into the epoxy resin composition various additives to improve the flame-retardancy of the resulting laminate. Many types of flame retardant additives have been suggested, but the additives which are most widely used commercially are halogen-containing additives, such as tetrabromo-diphenylolpropane, or epoxy resins prepared by reacting diglycidyl ether of bisphenol-A with tetrabromodiphenylolpropane. Typically, in order to reach the desired fire retardancy level (V-0 in the standard xe2x80x9cUnderwriters Laboratoryxe2x80x9d test method UL 94), levels of such bromine-containing flame retardant additives are required which provide a bromine content of from 10 wt % to 25 wt % based on the total polymer weight in the product.
Although halogen-containing fire-retardant additives such as tetrabromodiphenylolpropane are effective, they are considered by some to be undesirable from an environmental standpoint, and in recent years there has been increasing interest in the formulation of halogen-free epoxy resins, which are able to meet the fire retardancy requirements.
Proposals have been made to use phosphorus-based flame retardants instead of halogenated fire retardants in epoxy resin formulations as described in, for example, EP A 0384939, EP A 0384940, EP A 0408990, DE A 4308184, DE A 4308185, DE A 4308187, WO A 96/07685, and WO A 96/07686. In these formulations a phosphorus flame retardant is pre-reacted with an epoxy resin to form a di- or multifunctional epoxy resin which is then cured with an amino cross-linker such as dicyandiamide, sulfanilamide, or some other nitrogen element-containing cross-linker to form the network.
There are some commercially available phosphorus-based fire retardant additives which may be useful for replacing halogen-containing fire-retardant additives. For example, Amgard(trademark) V19 and Antiblaze(trademark) 1045 (previously Amgard(trademark) P45) supplied by Albright and Wilson Ltd, United Kingdom, are commercially available phosphonic acid ester fire retardant materials. These phosphonic acid esters, may be solids or liquids.
Alkyl and aryl substituted phosphonic acid esters are compatible with epoxy resins. In particular lower (i.e., C1-C4) alkyl esters of phosphonic acid are of value because they contain a high proportion of phosphorus, and are thus able to impart good fire retardant properties upon resins in which they are incorporated. However, the phosphonic acid esters are not satisfactory as a substitute for halogenated flame retardants in epoxy resins for the production of electrical laminates, because the use of phosphonic acid esters in amounts sufficient to provide the necessary flame retardancy increases the tendency of the resulting cured epoxy resin to absorb moisture. The moisture absorbency of the cured laminate board is very significant, because laminates containing high levels of moisture tend to blister and fail, when introduced to a bath of liquid solder at temperatures around 260xc2x0 C., a typical step in the manufacture of printed wiring boards.
Another system, which utilizes a phosphorus-based flame retardant, is described in EP A 0754728. EP A 0754728 describes the production of a flame retardant epoxy resin system by blending an epoxy resin with a cyclic phosphonate as a flame retardant and incorporating the cyclic phosphonate into the cured resin. The epoxide resin and phosphonate mixture is crosslinked with a polyamine such as triethylamine, tetra amine, polyamido amines, multi basic acids or their anhydrides for example phthalic anhydride or hexahydrophthalic anhydride. EP A 0,754,728 indicates that large quantities, such as in excess of 18 wt %, of the phosphorus additive are needed in order for the resin system to meet UL 94 V-0.
WO 99/00451 discloses flame retardant epoxy resin compositions utilizing phosphonic acid esters. WO 99/00451 discloses the reaction of a phosphonic acid ester with an epoxy resin in the presence of a catalyst and a nitrogen-containing crosslinking agent. The crosslinking agent has an amine functionality of at least 2 and is preferably dicyandiamide. The epoxy resins described in WO 99/00451 have improved flame retardant properties at low levels of phosphonic acid ester flame retardant. However, there is still a need in the industry for a flame retardant epoxy resin with improved Tg and flame retardant properties.
As aforementioned, halogen-containing phenol compounds such as tetrabromobisphenol-A (TBBA) are well known materials used in epoxy resins, specifically for use in the manufacture of FR-4 laminates for printed circuit boards. Halogen-containing compounds, specifically bromine-containing materials have the disadvantages of corrosive acidic components, e.g. HBr, released at high temperatures. It would be desirous to provide a non-halogenated material as a fire retardant additive to replace halogen-contaning phenol compounds such as TBBA.
The prior art describes the use of certain phosphorus element-containing compounds as crosslinking or curing agents for use with epoxy resins as a way to introduce a phosphorus element into epoxy resin systems. For example, U.S. Pat. Nos. 4,973,631 and 5,086,156 describe the use of difunctional phosphine oxide crosslinkers such as a triphenyl phosphine oxide epoxy curing agent and a trihydrocarbyl phosphine oxide epoxy curing agent having the following chemical structural formula: 
wherein Ar is an phenyl ring and the X moieties have an active hydrogen and include an amine, hydroxy, carboxy, anhydride and thiol moieties.
JP 61,134,395 [86,134,395] teaches the use of tris(3-hydroxyphenyl)phosphine oxide having the following chemical structural formula: 
as a crosslinking agent for epoxy resins.
The above-mentioned prior art compositions are not easily prepared and require exotic preparation procedures. It would be advantageous to provide a compound that can be derived from practical, industrial scale raw materials; and thus, would offer an economic advantage over the prior art processes.
The prior art describes the use of phosphates [OP(OR)3] and other compounds with Pxe2x80x94Oxe2x80x94C units in epoxy resin formulations. Such resins do not provide a satisfactory resistance to water uptake. It would be desirous to provide compounds such as triarylphosphine oxides which do not contain Pxe2x80x94Oxe2x80x94C bonds, and therefore, do not suffer the disadvantages of the prior art compounds having Pxe2x80x94Oxe2x80x94C units. It would be desirous to provide phosphorus compounds with Pxe2x80x94C linkages having superior resistance to water uptake when compared to phosphorus compounds with Pxe2x80x94Oxe2x80x94C units.
Phosphorus-containing compounds have been used heretofore in a variety of epoxy-based polymer systems including for example FR-4 laminates for printed circuit boards, container coatings, as well as high-molecular weight epoxy-based thermoplastics. A driving force for this work has been the search for non-halogenated alternatives to brominated epoxy resins. For a variety of reasons, brominated flame retardants have been deemed environmentally unsound and the use of brominated flame retardants in flame retardant epoxy resins applications has been continually threatened by legislation. Thus, there is a need to address this issue.
Commercially available non-brominated flame retardants have been used as replacements for terabromobisphenol-A (TBBA) in FR-4 laminates for printed circuit boards. Although most of these materials are found to meet the requirements for ignition and heat resistance, excessive moisture uptake in the board is found to be the main problem with their use. The presence of excess moisture cause the laminate boards to fail blister resistance testing. Phosphorus-containing species (phosphates, phosphonates, and phosphoramides) that contained Pxe2x80x94O and Pxe2x80x94N bonds, are believed to be the cause of excessive moisture uptake due to their polarity. Thus, it is desired to provide materials that contain Pxe2x80x94C bonds instead of Pxe2x80x94O or Pxe2x80x94N bonds in FR-4 laminate applications. It would be advantageous if such materials would reduce the amount of water absorbed by the finished laminate board. It is desired to provide compounds, which improve resistance to moisture uptake in the laminate boards, while maintaining the requirements of excellent Tg and ignition resistance.
The present invention is directed to epoxy resins which meet the desirable standards of fire retardancy without the need for halogen-containing flame retardants, or at least employing significantly lower levels of such halogen-containing flame retardants than is conventional in the art.
One aspect of the present invention is directed to new phosphorus element-containing crosslinking agents for epoxy resin compositions, more particularly crosslinking agents based on an isomeric mixture of two or more isomers of tris(2-hydroxyphenyl)phosphine oxides having the general chemical structure of Formula I as follows: 
wherein R is independently a hydrogen or a C1-C10 alkyl group such as methyl, ethyl, propyl, butyl, etc.
Another aspect of the present invention is directed to curable epoxy resin compositions containing a curable epoxy resin and, as a phosphorus element-containing crosslinking agent, an effective curing amount of a tris(2-hydroxyphenyl)phosphine oxide having the general chemical structure of Formula I as follows: 
wherein R is independently a hydrogen or a C1-C10 alkyl group such as methyl, ethyl, propyl, butyl, etc.
Yet another aspect of the present invention is directed to preparing a non-halogenated, ignition resistant epoxy resin formulation useful for making laminates for printed wiring boards and composite materials using the above phosphorus element-containing crosslinking agent of Formula I.
Hydroxyphenylphosphine oxides, such as those having the above general Formula I, which contain less hydrophilic phosphorus-phenyl units, are advantageously used for the synthesis of epoxy-based thermosets. Compounds of Formula I can be incorporated into epoxy-based polymers via covalent linkages. Improvements in glass transition temperatures and ignition resistance are realized when the resultant phosphorus-containing polymers are compared to conventional bisphenol-A based epoxy thermophaucs. The pbenol-functionalized phosphine oxide species of the present invention may be useful as replacements for tetrabromobisphenol-A in FR-4 laminates applications.
It is an objective of the present invention to provide a cross-linking agent which yields superior performance when compared to the previously known commercially available non-halogenated materials.
Still another aspect of the present invention is directed to a compound which ncludes the glycidyl ether derivatives of Formula I, shown as the following chemical tructural Formula V: 
wherein R is as defined above.
Yet another aspect of the present invention is directed to epoxy vinyl ester resins produced by reacting the compound of Formula V above with carboxylic acids which contain an unsaturated functionality to produce the compounds as shown in the following chemical structure of Formula VIII: 
wherein R is defined above and R1 is a moiety containing an unsaturated functionality such as a moiety having the following formula: 
where Rxe2x80x3 is a hydrogen or an alkyl group from C1-C20 and Rxe2x80x2xe2x80x3 may be a hydrogen, an alkyl group from C1-C20, or a carboxylic acid group or ester derivatives thereof. For example Rxe2x80x2 may be a methacrylate, acrylate, maleate, fumarate and the like.
In its broadest scope, the present invention is directed to a new phosphorus element-containing crosslinking agent advantageously used in epoxy-based formulations for preparing a flame retardant epoxy resin composition substantially free of halogen. The cross linkers of the present invention are preferably used as curing agents for epoxy formulations used in FR-4 laminates for printed circuit boards. Generally, the flame retardant composition includes (I) a non-halogenated epoxy resin material and (II) a phosphorus element-containing crosslinking agent.
A resin which is xe2x80x9csubstantially free of halogenxe2x80x9d means that the resin is completely free of halogen, i.e. 0% halogen, or that the resin contains some minor amount of halogen that does not affect the properties or performance of the resin, and is not detrimental to the resin. Usually resins which have not been halogenated with a halogen-containing compound are xe2x80x9csubstantially free of halogenxe2x80x9d.
xe2x80x9cSubstantially free of halogenxe2x80x9d, therefore, herein generally means that the resin contains less than 10 weight percent, preferably less than about 5 weight percent, more preferably less than about 1 weight percent, even more preferably less than 0.5 weight percent and most preferably zero weight percent of a halogen in the resin composition.
In order to obtain satisfactory ignition resistance, i.e. flame retardancy, and still obtain the benefit of resistance to water absorption, it is important that the amount of the phosphorus element in the epoxy resin composition is from about 0.2 wt % to about 5 wt %, preferably from about 0.5 wt % to about 3.5 wt %, even more preferably from about 1 wt % to about 3 wt %, and most preferably from about 1.5 wt % to about 2.8 wt %, based on the total of the solid resin composition.
The phosphorus element-containing crosslinking agent (II) of the present invention is preferably one or more compounds, or an isomeric mixture of two or more isomer compounds, having the following general chemical structural Formula I: 
wherein R is independently a hydrogen or a C1-C10 alkyl group including for example, methyl, ethyl, propyl, butyl, etc.
Specific examples of the crosslinkers (II) of the present invention may include those having the following chemical structural Formulas II-IV: 
The compounds of Formulas III and IV above are specific compounds wherein a CH3 group is in the 5 position of the phenyl ring as shown in Formula III or wherein a hydrogen is in the 3, 4, 5, and 6 positions of the phenyl ring as shown in Formula IV; and such compounds are described in Synthetic Communications, 1987, 17(4), 465-468. Heretofore, the compounds of Formulas III and IV have not been used as crosslinkers for epoxy resins, particularly for making flame retardant compositions.
The crosslinkers of the above Formula II are preferably used in the present invention. Formula II represents an isomeric mixture of two or more different triaryl phosphine oxide isomer compounds wherein the R group is a methyl and the methyl can be in the 3, 4, 5 or 6 position in each of the phenyl rings. The isomeric mixture of Formula II is new. A more preferred embodiment of the isomeric mixture of Formula II used in the present invention is when the mixture contains at least one of the isomers having at least one 2-hydroxy-4-methyl phenyl moiety and/or at least one of the isomers having at least one 2-hydroxy-5-methylphenyl moiety.
The ratio of 2-hydroxy-4-methylphenyl moiety to the 2-hydroxy-5-methyl phenyl moiety is generally dependent on the moiety ratios of the starting precursor; and the ratio can range from 99:1 to 1:99 and preferably, the ratio is from about 70:30 to about 30:70.
The cross-linker (II) of the present invention is preferably used in the epoxy resin composition in an amount of from about 50% to about 150% of the stoichionietric amount needed to cure the epoxy resin and more preferably from about 75% to about 125% of the stoichiometric amount needed to cure the epoxy resin, even more preferably from about 85% to about 110% of the stoichiometric amount needed to cure the epoxy resin.
Formulations that use the crosslinking agent of Formula II yield laminates with excellent glass transition temperatures (Tg) of from about 50xc2x0 C. to about 180xc2x0 C. and an ignition resistance of V-0/N-1.
Formulations made using the crosslinker of Formula II yield laminates have a Tg of preferably greater than 120xc2x0 C., more preferably greater than 135xc2x0 C., and most preferably greater than 150xc2x0 C. The crosslinker of Formula II also provides good resistance to water uptake for a phosphorus-containing additive/comonomer, with a moisture pickup of preferably less than about 1%, more preferably less than about 0.5%, and most preferably less than about 0.3% after about 40 minutes exposure time in a steam autoclave (known in the industry as the xe2x80x9cpressure cooker testxe2x80x9d). Thus, compounds of the general chemical structural Formula I, specifically the compound of Formula II, are superior to other known phosphorus-containing compounds used to prepare non-halogenated FR-4 laminates for printed circuit boards.
The compounds of Formulas I-IV may be prepared by a method described in Synthetic Communications, 1987, 17(4), pp. 465-468, incorporated herein by reference. Generally, the method of preparation of the compounds of the present invention involves the reaction of a corresponding precursor such as a triarylphosphate, with lithium diisopropyl amide in tetraydrofuran (THF) solvent at xe2x88x9278xc2x0 C. For example, commercially available triarylphosphate precursors such as tris(p,m-cresyl)phosphate and triphenylphosphate can be used as starting materials to prepare, on an industrial scale, the compounds of Formula II and Formula IV, respectively.
The crosslinker of Formula II is preferably soluble in aprotic and/or protic solvents commonly used for making epoxy resin formulations, particularly for use in laminate applications. The solvents include for example acetone, methylethyl ketone, methanol, ethanol, Dowanol PM*, N-methylpyrrolidone, dimethylforamide and the like. The solubility of the crosslinker is preferably greater than 30% solids content, more preferably greater than 40% solids contents and most preferably greater than 50% solids contents. The compounds of Formulas III and IV are high melting crystalline solids which are useful as crosslinking agents in other applications such as coatings, adhesives and composites.
An additional embodiment of the present invention is a compound which includes the glycidyl ether derivatives of Formula I, shown as the following chemical structural Formula V: 
wherein R is as defined above.
The compounds of Formula V can be produced by epoxidizing the compounds of Formula I using well known techniques in the art such as reacting the compounds of Formula I with an epihalohydrin such as epichlorohydrin to prepare the epoxy resin.
The resulting epoxy resin of Formula V can be advantageously used per se as an epoxy resin base material, such as those described herein as the non-halogenated epoxy resin materials of component (I), for making a flame retardant composition which is curable with any curing agent. More particularly, the curing agent used can also be the phosphorus element-containing crosslinking agent (II) disclosed herein as Formula I. In another embodiment, the compound of Formula V may also be used in combination with other phosphorus element-containing compounds. The combination of the compounds of Formula V and Formula I produce curable compositions which can be used in applications such as flame retardant coatings, laminates, composites and adhesives.
More beneficially, the compounds of Formula V can be used in laminate applications to produce laminates which have a reduced water up-take because of the Pxe2x80x94C bonds as opposed to those compounds having a Pxe2x80x94Oxe2x80x94C bonds.
In still another embodiment of the present invention, the previously described epoxy resin compound of Formula V can be further reacted with a carboxylic acid material which contains an unsaturated functionality to make a vinyl ester resin as shown in the following chemical structure of Formula VIII: 
wherein R is defined above and R1 is a moiety containing an unsaturated functionality such as a moiety having the following formula: 
where Rxe2x80x3 is a hydrogen or an alkyl group from C1-C20 and Rxe2x80x2xe2x80x3 may be a hydrogen, an alkyl group from C1-C20, or a carboxylic acid group or ester derivatives thereof. For example R may be a methacrylate, acrylate, maleate, fumarate and the like.
Preferably, the epoxy vinyl ester resins of Formula VIII is produced by reacting a carboxylic acid material having an unsaturated functionality such as acrylic acid, methacrylic acid, maleic acid, fumaric acid, and the like.
The amount of carboxylic acid material used is sufficient to completely react the acid functionality with all of the epoxy groups, i.e., stoichiometric amounts using techniques well known in the art. The epoxy vinyl ester resins may also be used with commonly reactive diluents such as styrene, divinyl benzene, and p-methyl styrene and other reactive diluents known in the art. The diluents are used for ease of fabricating, for example, composites; and for improving the properties of the final composites produced.
The compounds of Formula VIII are advantageously used to produce compositions which can be used in applications such as flame retardant coatings, laminates, composites and adhesives.
The crosslinking agent (also referred to as a xe2x80x9chardenerxe2x80x9d or xe2x80x9ccuring agentxe2x80x9d herein) of the present invention may be used alone, or in combination with one or more second different crosslinking agents, i.e., a co-crosslinking agent (VII). For example, the co-crosslinking agent used in the present invention may be selected from a multifunctional phenolic crosslinker, a nitrogen-containig crosslinker, styrenic maleic anhydride (SMA) or other phosphorus element-containing compounds.
In one optional embodiment, the composition of the present invention may include, as an additional crosslinking agent or co-crosslinker (VII), a multi-functional phenolic cross-linker which contains at least two or more functionalities. Examples of such phenolic crosslinking agents are described in co-pending U.S. Patent Application entitled, xe2x80x9cFlame Retardant Phosphorus Element-Containing Epoxy Resin Compositionsxe2x80x9d U.S. Pat. No. 6,645,631, filed of even date herewith, and incorporated by reference. For example, the multi-functional phenolic cross-links are preferably novolacs or cresol novolacs obtained by the condensation of phenols, cresols, xylenols or other alkyl phenols with formaldehyde. In the present invention, the resoles may also be used as the multi-functional phenolic cross-linker.
Preferably, if a multi-functional phenolic cross-linker is used in the present invention, the cross-linker has the following chemical structural Formula VI: 
wherein xe2x80x9cRxe2x80x9d is hydrogen or a C1-C3 alkyl, e.g., methyl and xe2x80x9cnxe2x80x9d or an integer from 1 to 10. xe2x80x9cnxe2x80x9d preferably has an average value of from 0 to 5.
In another embodiment, the composition of the present invention may include as an additional crosslinking agent, a nitrogen-containing cross-linker (VII), used as a subsidiary cross-linker in addition to the phosphorus element-containing cross-linker (II), and with or without the multi-functional phenolic cross-linker. The nitrogen-containing co-crosslinking agent preferably has an amine functionality of at least 2. Suitable multi-functional cross-linkers are described in numerous references such as Vol. 6 Encyclopedia of Poly. Sci. and Eng., xe2x80x9cEpoxy resinsxe2x80x9d at 348-56 (J. Wiley and Sons 1986). Examples of suitable nitrogen-containing co-crosslinkers useful in the present invention may be found in WO 99/00451, incorporated herein by reference; and include for example, include polyamines, polyamides, sulpfhanilamide, diaminodiphenylsulfone and diaminodiphenyl methane and dicyandiamide, substituted dicyandiamide, 2,4-diamino-6-phenyl-1,3,5-triazin. When a nitrogen-containing crosslinker is used in the present formulation, the preferred nitrogen-containing co-crosslinkers are dicyandiamide, sulfanilamide and, 2,4-diamino-6-phenyl-1,3,5-triazine, more preferable sulfanilamide issued.
Another preferred embodiment of co-crosslinkers (VII) useful in the present invention are described in U.S. Pat. No. 6,613,839, entitled Latent Catalysts for Epoxy Curing Systems, issued Sep. 2, 2003, to Gan et al; incorporated herein by reference; and which include, for example, copolymers of styrene and maleic anhydride having a molecular weight (Mw), in the range of from 1500 to 50,000 and an anhydride content of more than 15 percent. Commercial examples of these materials include SMA 1000, SMA 2000, and SMA 3000 having styrene-maleic anhydride ratios of 1:1, 2:1, and 3:1, respectively, and having molecular weights ranging from 6,000 to 15,000; and which are available from Elf Atochem S.A.
When one or more co-crosslinkers are used in the present invention, the co-crosslinkers are present in an amount to cross-link less than 40% of stoichiometric amount needed to cure the epoxy resin. Preferably, the amount of the co-crosslinking agent in the epoxy resin is from 0 to 40% of the stoichiometric quantity needed to cure the epoxy content of the epoxy resin in the formulation.
The non-halogenated epoxy resin material (I) of the present invention may be selected from: (A) a non-halogenated epoxy resin; (B) a non-halogenated, phosphorus element-containing epoxy resin; (C) a mixture of:(1) a non-halogenated, non-phosphorus element-containing epoxy resin, and (2) a phosphorus element-containing compound; (D) the reaction product of: (1) a non-halogenated epoxy resin; and (2) a phosphorus element-containing compound; or (E) a combination of two or more of components (A) to (D).
Generally, the non-halogenated epoxy resin material (I) used in the present invention is a material which possesses on average more than 1 and preferably at least 1.8, more preferably at least 2 epoxy groups per molecule. In the broadest aspect of the present invention, the epoxy resin material may be any saturated or unsaturated aliphatic, cycloaliphatic, aromatic or heterocyclic compound which possesses more than one 1,2-epoxy group.
The non-halogenated epoxy resins (A) useful in the present invention include polyepoxides. The polyepoxide compound useful in the present invention is suitably a compound which possesses an average of from 1.9 to 2.1 1,2-epoxy groups per molecule. In general, the polyepoxide compound is saturated or unsaturated aliphatic, cycloaliphatic, aromatic or heterocyclic compound which possesses more than one 1,2-epoxy group. The polyepoxide compound can be substituted with one or more substituents which are non reactive with the isocyanate groups such as lower alkyls and halogens. Such polyepoxide compounds are well known in the art.
Illustrative polyepoxide compounds useful in the practice of the present invention are described in the Handbook of Epoxy Resins by H. E. Lee and K. Neville published in 1967 by. McGraw Hill, New York and U.S. Pat. No. 4,066,628. Examples of suitable aromatic polyepoxides are bisphenol-A, bisphenol-F, bisphenol-AD, bisphenol-S, tetramethyl bisphenol-A, tetramethyl bisphenol-F, tetramethyl bisphenol-AD, tetramethyl bisphenol-S, tetrabromobisphenol-A, tetrachlorobisphenol-A, biphenols such as 4,4-biphenol or 3,3xe2x80x2,5,5xe2x80x2-tetramethyl-4,4xe2x80x2-biphenol, and dihydroxynaphthalene. Examples of suitable aliphatic polyepoxides are diglycidyl esters of hexahydrophthalic acid and diglycidyl esters of dicarboxylic acids, epoxidized polybutadien epoxidized soyabean oil, and epoxidized diols. Cycloaliphatic epoxides include, for example, 3,4-epoxy-6-methylcyclohexyl carboxylate and 3,4-epoxycyclohexyl carboxylate. Examples of heterocyclic epoxy compounds are diglycidylhydantoin or triglycidyl isocyanurate (TGIC).
Preferred polyepoxides are glycidyl compounds of bisphenol-A, of bisphenol-F, of tetrabromobisphenol-A and of 3,3xe2x80x2,5,5xe2x80x2-tetramethyl-4,4-biphenol. Mixtures of any two or more polyepoxides can also be used in the practice of the present invention
Examples of a non-halogenated, phosphorus element-containing epoxy resin (B) useful in the composition of the present invention include:
1. Bisglycidylphenylphosphate as illustrated by the following chemical formula: 
2. The epoxy phosphine oxides described in S. V. Levchik, G. F. Levchik, A. V. Antonov, M.Yu. Yablokova, O. I. Tuzhikov, O. O. Tuzhikov and L. Costa, 1999 including for example the following compounds: 
wherein R is as previously defined; and
3. Aryl phosphinate epoxy ether 10-(2xe2x80x2,5xe2x80x2-bis(glycidyloxy)phenyl)-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DHQEP) as illustrated by the following chemical formula: 
The synthesis of DHQEP as an effective flame retardant and thermal stabilizer for epoxy resins is described in Cho, Ching-sheng/Chen, Leo-wang/Chiu, Yie-shun; Polymer Bulletin 41, 45-52 (1998),
In another preferred embodiment the epoxy resin material (I) used in the present invention may (C) a mixture of (1) an epoxy compound containing at least two epoxy groups, and (2) a phosphorus-containing compound. It is possible to add the phosphorus containing compound (C) and the epoxy resin compound (C) to the overall composition of the present invention in order to form the epoxy resin material (I) in-situ.
In another preferred embodiment, the epoxy resin material (I) which is added to the overall composition of the present invention, may be (D) the reaction product of (1) an epoxy resin and (2) a phosphorus-containing D1 compound capable of reacting with the epoxy resin.
The aforementioned epoxy resin (C1) or (D1) is preferably one which has no lower alkyl aliphatic substituents, for example the glycidyl ether of a phenol novolac, or the glycidyl ether of bisphenol-F.
The most preferred epoxy resins (C1) or (D1) are epoxy novolac resins (sometimes referred to as epoxidised novolac resins, a term which is intended to embrace both epoxy phenol novolac resins and epoxy cresol novolac resins). Such epoxy compounds have the following general chemical structural Formula VII: 
wherein xe2x80x9cRxe2x80x9d a is hydrogen or a C1-C3 alkyl, e.g., methyl and xe2x80x9cnxe2x80x9d is 0 or an integer from 1 to 10. xe2x80x9cnxe2x80x9d preferably has an average value of from 0 to 5.
Epoxy novolac resins (including epoxy cresol novolac resins) are readily commercially available, for example under the trade names D.E.N.(trademark) (Trademark of The Dow Chemical Company), Quatrex(trademark), Tactix(trademark). The materials of commerce generally comprise mixtures of various species of the above formula and a convenient way of characterizing such mixtures is by reference to the average, nxe2x80x2, of the values of n for the various species. Preferred epoxy novolac resins for use in accordance with the present invention are those in which nxe2x80x2 has a value of from about 2.05 to about 10, more preferably from about 2.5 to about 5.
The phosphorus-containing compound or monomer (C2) or (D2) contains some reactive groups such as a phenolic group, an acid group, an amino group, an acid anhydride group, or a phosphinate group (Pxe2x80x94H) which can react with the epoxy groups of the epoxy resin compound (C1) or (D1).
In a most preferred embodiment of the present invention the phosphorus-containing monomer (C2) or (D2) used in the present invention is for example, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide such as xe2x80x9cSanko-HCAxe2x80x9d commercially available from SANKO of Japan or xe2x80x9cStruktol Polydis PD 3710xe2x80x9d commercially available from Schill-Seilacher of Germany; 10-(2xe2x80x2,5xe2x80x2-dihydroxyphenyl)-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (HCA-HQ); bis(4-hydroxyphenyl)phosphine oxide; tris(2-hydroxyphenyl)phosphine oxide; dimethyl-1-bis(4-hydroxyphenyl)-1-phenylmethylphonate; an isomeric mixture of tris(2-hydroxymethylphenyl)phosphine oxides; or tris(2-hydroxy-5-methylphenyl) phosphine oxide; and mixtures thereof.
The phosphorus-containing compound (C2) or (D2) of the present invention may contain on average one or more than one functionality capable of reacting with the epoxy groups. Such phosphorus-containing compound preferably contains on average 0.9 to 3 functional groups capable of reacting with epoxy resin.
The compositions of the present invention preferably contain a catalyst (III) capable of promoting the reaction of the cross-linker of the present invention, and/or the phosphorus-containing compounds with the epoxy resin and promoting the curing of the epoxy resin.
Examples of suitable catalyst materials useful in the present invention include for example compounds containing or amine, phosphine, ammonium, phosphonium, arsonium or sulfonium moieties. Particularly preferred catalysts are heterocyclic nitrogen-containing compounds.
The catalysts (III) (as distinguished from cross-linkers) preferably contain on average no more than about 1 active hydrogen moiety per molecule. Active hydrogen moieties include hydrogen atoms bonded to an amine group, a phenolic hydroxyl group, or a carboxylic acid group. For instance, the amine and phosphine moieties in catalysts are preferably tertiary amine or phosphine moieties; and the ammonium and phosphonium moieties are preferably quaternary ammonium and phosphonium moieties. Among preferred tertiary amines that may be used as catalysts are those mono- or polyamines having an open-chain or cyclic structure which have all of the amine hydrogen replaced by suitable substituents, such as hydrocarbyl radicals, and preferably aliphatic, cycloaliphatic or aromatic radicals. Examples of these amines include, among others, 1,8-diazabicyclo (5.4.0)undec-7-en (DBU), methyl diethanol amine, triethylamine, tributylamine, dimethyl benzylamine, triphenylamine, tricyclohexyl amine, pyridine and quinoline. Preferred amines are the trialkyl, tricycloalkyl and triaryl amines, such as triethylamine, triphenylamine, tri-2,3-dimethylcyclohexyl)amine, and the alkyl dialkanol amines, such as methyl diethanol amines and the trialkanolamines such as triethanolamine. Weak tertiary amines, for example, amines that in aqueous solutions give a pH less than 10 in aqueous solutions of 1 M concentration, are particularly preferred. Especially preferred tertiary amine catalysts are benzyldimethylamine and tris-dimethylaminomethyl) phenol.
Examples of suitable heterocyclic nitrogen-containing catalysts (III) include those described in U.S. Pat. No. 4,925,901. Preferable heterocyclic secondary and tertiary amines or nitrogen-containing catalysts which can be employed herein include, for example, imidazoles, benzimidazoles, imidazolidines, imidazolines, oxazoles, pyrroles, thiazoles, pyridines, pyrazines, morphoimes pyridazines, pyrimidines, pyrrolidines, pyrazoles, quinoxalines, quinazolines, phthalozines, quinolines, purines, indazoles, indoles, indolazines, phenazines, phenarsazines, phenothiazines, pyrrolines, indolines, piperidines, piperazines and combinations thereof. Especially preferred are the alkyl-substituted imidazoles; 2,5-chloro-4-ethyl imidazole; and phenyl-substituted irnidazoles, and mixtures thereof. Even more preferred are N-methylimidazole; 2-methylimidazole; 2-ethyl4-methylimidazole; 1,2-dimethylimidazole; and 2-methylimidazole. Especially preferred is 2-phenylimidazole.
Preferably a Lewis acid (IV) is also employed in the composition of the present invention, especially when the catalyst (III) is, particularly a heterocyclic nitrogen-containing compound.
Examples of heterocyclic nitrogen-containing catalysts(III) which are preferably used in combination with Lewis acids (IV) are those described in EP-A-526488, EP A 0458502 and GB A 9421405.3. Examples of suitable Lewis acids useful in the present invention include halides, oxides, hydroxides and alkoxides of zinc, tin, titanium, cobalt, manganese, iron, silicon, aluminium, and boron, for example Lewis acids of boron, and anhydrides of Lewis acids of boron, for example boric acid, metaboric acid, optionally substituted boroxines (such as trimethoxyboroxine), optionally substituted oxides of boron, alkyl borates, boron halides, zinc halides (such as zinc chloride) and other Lewis acids that tend to have a relatively weak conjugate base. Preferably the Lewis acid is a Lewis acid of boron, or an anhydride of a Lewis acid of boron, for example boric acid, metaboric acid, an optionally substituted boroxine (such as trimethoxy boroxine, trimethyl boroxine or triethyl boroxine), an optionally substituted oxide of boron, or an alkyl borate. The most preferred Lewis acid is boric acid. These Lewis acids are very effective in curing epoxy resins when combined with the heterocyclic nitrogen-containing compounds, referred to above.
The Lewis acids and amines can be combined before mixing into the formulation or by mixing with the catalyst in-situ, to make a curing catalyst combination. The amount of the Lewis acid employed is preferably at least 0.1 moles of Lewis acid per mole of heterocyclic nitrogen compound, more preferably at least 0.3 moles of Lewis acid per mole of heterocyclic nitrogen-containing compound.
The formulation preferably contains no more than 3 moles of Lewis acid per mole of catalyst and more preferably contains no more that 2 moles of Lewis acid per mole of catalyst The total amount of the catalyst is from about 0.1 wt % to about 3 wt %, based on the total weight of the Composition, preferably from about 0.1 wt % to about 2%.
The compositions of the present invention may also optionally contain one or more additional flame retardant additives (V), including for example, red phosphorous or liquid or solid phosphorus-containing compounds, for example, ammonium polyphosphate, a phosphite, or phosphazenes, nitrogen-containing fire retardants and/or synergists, for example melamines, urea, cynamide, guanidine, cyanuric acid, isocyanuric acid and derivatives of those nitrogen-containing compounds, halogenated flame retardants, halogenated epoxy resins (especially brominated epoxy resins) synergistic phosphorus-halogen containing chemicals or compounds containing salts of organic acids, inorganic metal hydrates, boron or antimony. Examples of suitable additional flame retardant additives are given in a paper presented at xe2x80x9cFlame retardantsxe2x80x94101 Basic Dynamicsxe2x80x94Past efforts create future opportunitiesxe2x80x9d, Fire Retardants Chemicals Association, Baltimore Marriot inner harbour hotel, Baltimore Md., Mar. 24-27 1996.
When additional fire retardants which contain a phosphorus element are present in the composition of the present invention, the phosphorus element-containing fire retardants are generally present in amounts such that the total phosphorous content of the epoxy resin composition is from about 0.2 wt % to about 5 wt %.
Other non-epoxy, phosphorus additives which can be used in the present invention may include for example phosphorus-guanamine having the following chemical structure: 
where X is CR3R4-(CR1R2)nxe2x80x94CR5R6 or o-phenylidene, n is 0 or 1 R1-R8 may be the same or different and represent H, CH3, C2H5.
The phosphorus-quanamine above is found to be a highly suitable alternative to brominated epoxy resins for printed circuit board (PCB) applications as described in Buser, Antonius Johannes Wilhelmus/Schutyser, Jan Andre Jozef, Akzo Nobel N.V. European Pat. 0825217 A1 (25.02.1998) and Buser, Antonius Johannes Wilhelmus/Schutyser, Jan Andre Jozef, Akzo Nobel N.V. European Pat. WO 98/07731 (26.02.98).
Another phosphorus additive useful in the present invention is xe2x80x9cAntiblaze #1045xe2x80x9d available from Albright and Wilson. Antiblaze #1045 is a cyclic phosphonate dimer containing 20.8% phosphorous and has the following structure: 
The following amino triazine phosphates are also phosphorus additives which may be used in the present invention: 
where R1, R2 and R3 is independently H or a C1-C40 alkyl group such as methyl.
The composition of the present invention may also include other inorganic fillers (VI), for example fillers such as aluminum trihydroxide e.g. xe2x80x9cMartinal OL-104xe2x80x9d from Martinswerke GmbH; magnesium hydroxide, e.g. xe2x80x9cZerogen 30xe2x80x9d available from Martinswerke GmbH; and ammonium polyphosphate, e.g. xe2x80x9cExolit 700xe2x80x9d available from Clariant GmbH. The aforementioned inorganic fillers can be added to the composition of the present invention to improve the flame retardant properties of the composition.
Solvents (VIII) may also optionally be used in the composition of the present invention. When a solvent is used it may include for example, propylene glycolmethylether (Dowanol PM(trademark)), methoxypropylacetate (Dowanol PMA(trademark)), methylethylketone (MEK), acetone, methanol, and combinations thereof When a solvent is used in the present invention, the amount of solvent present in the epoxy resin composition of the present invention is generally form 0 wt % to about 50 wt %; preferably from about 10 wt % to about 40wt % and more preferably from about 10 wt % to about 35 wt %, depending on the end use application of the epoxy resin composition.
Optionally, in some applications it may be desirable to add a small amount of a halogenated epoxy resin (IX, provided the halogenated epoxy resin is added in an amount such that the halogen content of the overall epoxy resin composition of the present invention is less than 10 weight percent.
The compositions of the present invention can be produced by mixing all the components together in any order. Preferably, the compositions of the present invention can be produced by preparing a first composition comprising the epoxy resin, and the second composition comprising the crosslinking agent. Either the first or the second composition may also comprise a phosphorus-containing compound a curing catalysts a multi-functional phenolic co-crosslinking agent and/or a nitrogen-containing co-crosslinking agents. All other components may be present in the same composition, or some may be present in the first, and some in the second. The first composition is then mixed with the second composition, and cured to produce a fire retardant epoxy resin.
The compositions of the present invention can be used to make composite materials by techniques well known in the industry much as by pultrusion, moulding, encapsulation, or coating.
The present invention is particularly useful for making B-staged prepregs and laminates by well known techniques in the industry.
A number of preferred embodiments of the present invention are illustrated, in the following specific Examples.